Polyolefins: 50 years after Ziegler and Natta I

This anthology summarizes contributions to Ziegler–Natta catalysis that have led through novel experimental methods to well-founded results, to continuing conclusions, and to increasing knowledge about the course of this catalysis. The complex and manifold mechanism has been elucidated in extensive kinetic studies in a half continuous tank reactor, in a plug flow reactor with oligomer distribution analysis as an additional “measuring instrument,” by dynamic NMR spectroscopy with line shape analysis, through use of ¹³C-enriched reacting ethylene as a molecular surveyor, and by observation of the individual behavior of a large number of particles with supported catalysts by transmission electron microscopy and video microscopy. The synopsis of the results of the different experimental methods gives assurance that we are safely on the right path.

This chapter looks back at the fascinating history of isotactic polypropylene, the first man-made stereoregular polymer, from the largely serendipitous discovery to the modern technologies for the industrial production of reactor blends with high-yield Ziegler–Natta catalysts featuring highly controlled morphology. This is also the story of a great man, Giulio Natta, winner of the 1963 Nobel Prize in Chemistry, and his team of incredibly talented young coworkers at the Milan Polytechnic, who in just a few years at the end of the 1950s elucidated the structure of the new polymer and that of the novel TiCl₃-based catalysts leading to its formation. The pioneering studies that followed on chain microstructure and the origin of the stereocontrol, and the first educated guesses on the nature of the active species, are critically reviewed, and re-visited with the aid of modern experimental and computational tools and methods, to highlight the current picture of what still represents a most important and lively area of polymer science and organometallic catalysis.

Since the discovery of the ethene polymerization with transition metal catalysts of group IV of the periodic table in combination with aluminum alkyl compounds as cocatalysts at low pressures and moderately high temperatures by Ziegler and colleagues in 1953, this catalytic polymerization process has been developed over six decades in an outstanding way and is now a mature technology for the production of high-density polyethylene grades with excellent properties for wide fields of application. Today, super-active heterogeneous catalysts are available. The catalyst must be designed to achieve high activity over a long polymerization time, be able to control average molecular mass over a wide range using hydrogen, to copolymerize ethene with higher 1-olefins, and to produce an unimodal polymer with a relative narrow molecular mass distribution. It is of greatest importance to avoid overheating of the growing polymer particle, especially when the polymerization starts at the virgin catalyst particle. This is not easy to achieve because the polymerization process is highly exothermic. The transformation of a catalyst particle into a polymer grain can be described and is well understood by the microreactor model. The technical process can be divided into three clear distinguishable levels: the microscale, the mesoscale, and the macroscale. The microscale level comprises all processes inside and at the surface of the growing polymer particle, i.e., the microreactor behavior. The mesoscale level deals with all processes inside the three-phase reactor content comprising gas bubbles, hydrocarbon diluent and the solid growing polymer particles. It is important to achieve reproducible and stable conditions on the basis of a detailed chemical engineering on this mesoscale level throughout the reactor. If this is the case, then this polymerization process can be well controlled on the macroscale level, comprising the polymerization vessel as a whole. By controlling a limited number of process data, the slurry polymerization process can be operated with excellent stability over a long time and can be controlled within narrow ranges. The modern slurry technology process is very flexible in controlling product properties by using the cascaded reactor technology. This technology involves two or even three reactors operated in series under different process conditions. The catalyst is only introduced into the first reactor. The polymerizing particle then passes through all reactors, producing different types of macromolecules to form a polymer blend within each polymer grain. A further enormous advantage of this cascade technology is the high flexibility in product change and product development. Just by changing process parameters of the different reactors, products with different average molecular mass, different molecular mass distributions, different copolymer compositions, and different comonomer distributions can be produced without changing the catalyst system.

Since the discovery of electron donors for MgCl₂-supported Ziegler–Natta catalysts, donors have become key components for improving the stereospecificity and activity of these catalysts. Starting from benzoate for third-generation catalysts, the discovery of new donor structures has always updated the performance of Ziegler–Natta catalysts. Numerous efforts have been devoted since the early 1970s, in both industry and academy, not only for discovering new donors but also for understanding their roles in Zielger–Natta olefin polymerization. This chapter reviews the history of these efforts, especially after the twenty-first century. The first half of the chapter describes the history of catalyst developments, with special focus on industrialized donors, and then introduces recent trends in the development of new donors. The second half reviews historical progress in the mechanistic understanding of how donors improve the performance of Ziegler–Natta catalysts.

Kinetic investigations of olefin polymerization with Ziegler–Natta (ZN) catalysts provide information for understanding the mechanism of these reactions and are also necessary for development of industrial productions of polyolefins. In this chapter, the main kinetic features of olefin polymerization with heterogeneous ZN catalysts are considered as well as problems such as a deviation from the linear dependence of the rate of polymerization on monomer concentration, the hydrogen effect in ethene and propene polymerizations, and the comonomer effect, the natures of which are not yet completely clear and are discussed in the literature. For analysis of the kinetics and mechanism of olefin polymerization, data on the number of active centers and propagation rate constants are important. The main methods for determination of these kinetic parameters are discussed in this chapter. Data on the number of active centers in ZN catalysts of different composition are presented. On the base of these kinetic data, the hydrogen effect and the heterogeneity of active centers at propylene polymerization over ZN catalysts are analyzed.

The Phillips Cr/silica catalyst, discovered by Hogan and Banks at the Phillips Petroleum Company in the early 1950s, is one of the most important industrial catalysts for polyethylene production. In contrast to its great commercial success during the past half-century, academic progress regarding a basic understanding of the nature of the active sites and polymerization mechanisms is lagging far behind. During the last decade, increasing research efforts have been performed on the Phillips catalyst through various approaches, including spectroscopic methods, polymerization kinetics, heterogeneous model catalysts, homogeneous model catalysts, and molecular modeling. Much deeper mechanistic understanding, together with successive catalyst innovations through modifications of the Phillips catalyst, has been achieved.

Erratum to: Adv Polym Sci